Resins for extrusion coating on paper, board, aluminum etc., require broad MWD (molecular weight distribution) and low extractables. In extrusion coating applications the polymer is processed at high temperature conditions, typically above 280° C. and below 350° C. Broad molecular weight distribution (MWD) resins with a very high molecular weight fraction are used for good processability during coating (neck-in and drawdown balance). Low extractables are needed to reduce undesirable taste and odor issues in the final product, and to reduce smoke formation during the processing of the resin, especially during coating processes.
Typically low density polyethylene (LDPE) resins with broad MWD are made using autoclave reactors or a combination of autoclave and tube reactors. Broad MWD resins can be achieved in autoclave reactors by promoting long chain branching, and through the inherent residence time distribution, by which molecules will undergo shorter (low molecular weight) or longer (high molecular weight) growth paths. Broad MWD autoclave resins for LDPE are dominantly focused in two product density regimes, namely from 0.915 to 0.919 g/cc and from 0.919 to 0.924 g/cc. The invention in this document describes improved broad MWD tubular reactor products designed for the lower densities up to 0.919 g/cc.
The autoclave and tubular reactor systems differ in residence time distribution, which is typically more uniform for tubular reactors and dispersed for autoclave reactors. The uniform residence time in tubular reactors leads to narrower MWD; therefore very broad MWD can only be achieved by applying extremely differentiated polymerization conditions (e.g., WO 2013/078018), and/or application of a branching/cross-linking agent (e.g., U.S. Pat. No. 7,820,776). The use of extreme process conditions and/or branching/cross-linking agents can lead to high melt strength tubular low density polyethylene suitable for extrusion coating applications; however with elevated extractables versus autoclave products. These tubular polyethylenes will have a specific composition (e.g. density), and functionality as determined by the applied process conditions, type and level of branching agent and/or comonomer. Undesirable gels in the polymer can be an issue, resulting from the use of branching or cross-linking agents. Due to the difference in cooling capability between the tubular and the autoclave process, the conversion level ranges typically from <20% for the autoclave to >30% for the tubular process. This large difference in conversion level has a major impact on operation costs as well on polymer output and power consumption (to compress ethylene) per unit of polymer.
The U.S. Food and Drug Administration (FDA) regulations prescribe two hexane extractable limits for polyethylene in contact with food, namely a maximum of 5.5 weight percent (wt %) for general food contact and a maximum of 2.6 wt % for cook-in food contact applications. The manufacture of low density polyethylene (LDPE) that has both a broad MWD and low hexane extractables, but that is also suitable for extrusion coating applications, is difficult. For LDPE made in a high pressure (e.g., greater than or equal to (≥) 100 MPa) polymerization that comprises at least one tubular reactor with multiple reaction zones, the small molecules that are formed in the last reaction zones do not propagate, i.e., form into larger molecules, and are thus prone to migrate out of the bulk polymer. Formation of these small molecules in the last reaction zones is to be avoided, or at least reduced, to reduce the hexane extractables.
Other ethylene-based polymers and polymerizations are disclosed in WO Nos. 2007/110127; 2012/057975, 2012/084787, 2013/078018; 2013/078224; 2013/178241; 2013/178242; 2013/149698; 2013/132011, 2013/083285, and 2014/003837; US 2008/0242809, and EP 2 681 250 B1. There remains a need for tubular ethylene-based polymers with a density below 0.9190 g/cc, a broad MWD, a high G′, reduced extractable levels, and made at high ethylene conversion levels. These needs have been met by the following invention.